Hitherto, an editing system and the like have a structure that a video signal is accumulated in a hard disk unit. An apparatus of the foregoing type is enabled to reliably record/reproduce a high quality, large volume and high transferrate video signal by a structure arranged in such a manner that a disk array apparatus is formed by a plurality of hard disk units. The disk array apparatus forms a RAID (Redundent Array of Inexpensive Disks) having considerable redundancy.
FIG. 22 is a block diagram showing the schematic structure of the disk array apparatus forming the RAID. The disk array apparatus 1 inputs/outputs data D1 to and from a host through a disk-array controller 2. The disk-array controller 2 manages the operations of a plurality of hard disk units HDD1 to HDD5. The disk array apparatus 1 sequentially and circularly assigns, for example, one byte data (indicated with numerals 0, 1, 2, 3, . . .) forming data D1 to five hard disk units HDD0 to HDD4 except for the hard disk unit HDD5. Moreover, the disk array apparatus generates parity data P0, P1, . . . , composed of recovering data from data assigned to the hard disk units HDD0 to HDD4. Then, the disk array apparatus assigns parity data P0, P1, . . . , to the residual hard disk unit HDD5.
If any one of the hard disk units cannot easily reproduce data, the disk array apparatus 1 forming the RAID is arranged to obtain an exclusive OR of data reproduced by the other hard disk unit. Thus, the disk array apparatus is able to easily and quickly recover data. As a result, a reliable recording/reproducing system can be formed by that extent.
When a disk array apparatus of the foregoing type is applied to an editing apparatus or the like, video signals of a plurality of channels must be recorded/reproduced. On the other hand, hard disk units each having a usual input/output interface, such as a SCSI (Small Computer System Interface), are arranged to input/output data asynchronously with one another. Therefore, the hard disk units having the usual input/output interface have a problem in that a disk array apparatus of the foregoing type cannot easily be formed to record/reproduce a video signal without interruption. That is, recording/reproducing of a video signal without interruption requires continuity of the video signal to be maintained. To maintain the continuity, video signals of plural channels must synchronously be recorded/reproduced. The foregoing fact also applies to an audio signal which must be recorded/reproduced together with the video signal.
As a method of solving the above-mentioned problem, it might be considered feasible to employ a method using time slots for managing time to in parallel operating a plurality of disk array apparatuses.
That is, in a case where a system is formed in which video data can be input/output among six hosts when video data for one channel can be recorded by one disk array apparatus, for example, six disk array apparatuses are prepared. Moreover, the time base of video data of each channel is compressed to be simultaneously and in parallel assigned and output to the six disk array apparatuses. In addition, the overall operation is controlled as shown in FIG. 23 in such a manner that the operations are repeated in a cycle of a predetermined time T. Each time T is separated into pieces, the number of which is the same as that of the hosts so that time slots T1 to T6 are formed. The time slots T1 to T6 are sequentially assigned to the respective hosts. Then, video data items of the respective channels are input/output to and from the disk array apparatus in the corresponding time slots T1 to T6.
As a result, the respective disk array apparatuses receive, from the first to third hosts, data W1, W2 and W3 in the time slots T1, T2 and T3. Then, the disk array apparatus distributes respective data W1, W2 and W3 to the five hard disk units HDD0 to HDD4, and then records data in the following time slots T2, T3 and T4. At this time, the disk array apparatus generates parity data from the respective data W1, W2 and W3. The parity data are recorded on the residual hard disk unit HDD5 in the time slots T2, T3 and T4 (see FIGS. 23 (A) and 23 (B)).
When a read command has been input from the fourth host in the time slot T4, data R4 is read from each of the hard disk units HDD0 to HDD5. In the next time slot T5, the data R4 is output to the host (FIGS. 23 (B) and 23 (C)). It can therefore be considered that data of six channels indicated with numerals 1 to 6 can synchronously be recorded/reproduced.
However, a hard disk unit of the above-mentioned type sometimes encounters occurrence of a seek error. A hard disk unit having a usual input/output interface, such as the SCSI or the like, again performs seeking in this case, that is, performs retrial. If such a retrial operation is performed in any one of the hard disk units in, for example, the time slot T2, this hard disk unit cannot easily complete recording of data W1 in the time slot T2. Thus, recording of data W1 also requires the next time slot T3. Therefore, the foregoing hard disk unit cannot easily record data W2 which must be recorded inherently in the next time slot T3. As a result, a portion of the data W2 assigned to the foregoing hard disk unit is lost from the viewpoint of the overall system.
Even if the process is completed in the time slot, recording of an erroneous data for some reason or other cannot completely be prevented. Also in this case, a portion of data is undesirably lost. If data can correctly be recorded in the hard disk unit, a portion of data is sometimes lost attributable to a latter defect.
Although data, a portion of which has been lost, can be recovered by using data in another hard disk unit, the redundancy of the data above deteriorates as compared with other data items. As a result, the reliability of the system deteriorates.
In the foregoing case, a method may be considered in which the same data item is again resent from the host to recover lost data as is performed in a usual apparatus using the hard disk unit. However, difficulties are sometimes associated with resending of data when the method is applied to a process for recording/reproducing a video signal. Moreover, the load which must be borne by the host is increased.
If data, a portion of which has been lost, can correctly be recovered without any interruption of the operations for recording/reproducing continuous a video signal and an audio signal, deterioration in the redundancy can effectively be prevented. As a result, the reliability of the overall system can be improved by that extent. What is more, increasement of the load, which must be borne by the host to recover data, can effectively be avoided.
In view of the foregoing, according to the present invention, there is proposed a data recording/reproducing apparatus capable of recovering data, which cannot easily and correctly be recorded/reproduced, without any interruption of the operation for recording/reproducing continuous data.